In a time-of-flight mass spectrometer (which is hereinafter abbreviated to TOFMS), various ions that have been almost simultaneously accelerated by an electric field are introduced into a flight space formed within a flight tube. Those ions are subsequently separated into different kinds of ions having different masses (or m/z, to be exact) according to their time of flight, i.e. the time required for each ion to travel through the flight space until it reaches the detector. The detector continuously produces detection signals corresponding to the amount of the incoming ions. Therefore, after converting the time-of-flight to the mass, it is possible to create a mass spectrum with the abscissa axis as the mass axis and the coordinate axis as the signal intensity axis.
In the TOFMS, the flight distance of the ions can slightly change due to a mechanical expansion or contraction due to a temperature change of the flight tube. This leads to a variation in the time of flight of the ions having the same mass, which causes a shift of the mass axis of the mass spectrum. If the temperature change of the flight tube is large, the aforementioned shift of the mass axis may possibly exceed the specified mass accuracy of the apparatus. To avoid this situation, the flight tube of conventional types of TOFMS are contained in a vacuum chamber placed within a thermostatic bath (or temperature-controlled casing) with an aim to suppress the temperature change of the flight tube by controlling the temperature of the entire vacuum chamber. (For example, refer to Patent Documents 1 and 2.)
However, even if the temperature of the vacuum chamber is controlled, the temperature control of the vacuum chamber may be disordered by a sudden change in the ambient temperature or other factors, which can consequently cause a shift of the mass axis. Therefore, it is necessary to estimate, in real time, the shift length of the mass axis in some way and invite the users' attention if the shift is likely to exceed a tolerance level.
An appropriate method for estimating the shift length of the mass axis due to the aforementioned factors is to directly monitor the temperature of the flight tube and estimate the shift length of the mass axis from the monitored values. However, it is difficult to attach a temperature sensor to the flight tube to directly monitor its temperature, because flight tubes are generally used as an accelerating electrode for initially accelerating the ions and hence need to be supplied with a high voltage of several kV or higher and placed in a vacuum atmosphere within a vacuum chamber. Given this problem, a temperature sensor is normally attached on the external surface of the vacuum chamber exposed to the air inside the thermostatic bath, and the shift length of the mass axis is estimated from the temperature of the vacuum chamber monitored with the temperature sensor.
However, it is inevitable that the actual temperature change of the flight tube has a relatively large response delay from the monitored temperature of the vacuum chamber since the heat capacity of the flight tube is generally large and the thermal conductivity of the vacuum atmosphere is intrinsically low. If the shift length of the mass axis is determined on the assumption that the value monitored with the temperature sensor attached to the vacuum chamber equals the temperature of the flight tube, the determination result may be erroneous. Consequently, an analysis result that actually contains a significant mass shift may be mistaken for an accurate result and adopted. Conversely, an actually correct analysis result may be mistaken for a poorly accurate one and discarded.
For such problems, the applicant of the present patent application has proposed a new type of TOFMS in the Japanese Patent Application No. 2006-344370. In this TOFMS, a step response of the shift length of the mass axis, which occurs when the temperature of the vacuum chamber is changed in a step-like form, is measured beforehand, and parameters that express a transfer function based on this step response are stored in a memory unit. Using this transfer function stored in the memory unit and the monitored temperature of the vacuum chamber obtained in real time during the analysis, the current shift length of the mass axis is estimated. By this method, it is possible to estimate the shift length of the mass axis more precisely than ever before and invite users' attention by an annunciation unit if the shift length of the mass axis exceeds a tolerance level.
The shift length of the mass axis estimated in the previously described manner can also be used to correct the mass axis of the mass spectrum and thereby improve the mass accuracy of the spectrum. However, the required level of mass accuracy varies depending on the purpose of the analysis and other factors; it is in some cases necessary to achieve higher levels of mass accuracy that cannot be achieved by the mass axis correction based on the previously described estimating operation.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-170155    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-140064